1. Field of the Invention
The present invention relates to the protection of a secondary battery in the case where a charging current from a charger to a secondary battery is abnormally large (in case of an abnormal charging current).
2. Description of the Related Art
FIG. 2 shows a conventional protective IC and protective device against charge and discharge of a secondary battery. The conventional protective device includes a convention IC 21, a discharging current control switching element 16, a charging current control switching element 17 and a current fuse 22. The conventional protective IC 21 includes an overcharge detecting circuit 7, an overdischarge detecting circuit 8, an overcurrent detecting circuit 9, a level converting circuit 10, a NOR circuit 14 and so on. The protective IC 21 has a function of preventing a secondary battery 15 from being charged up to an excessive voltage by controlling the on/off operation of the discharging current control switching element 16 and the charging current control switching element 17 (an overcharge protective function), a function of preventing the secondary batter 15 from being discharged down to a too small voltage (an overdischarge protection function), and a function of preventing the secondary battery 15 from discharging an excessive current (an overcurrent protection function).
In order to realize a function of preventing an excessive charging current from flowing into the secondary battery 15 from the charger (an abnormal charging current protection function), the conventional protective device is designed such that a current fuse 22 is disposed between a secondary battery positive electrode connection terminal 2 and a charger positive electrode connection terminal 19 and the current fuse 22 is cut when a large charging current (abnormal charging current) flows.
In the case of using the conventional current fuse 22, there arises such a problem that the secondary battery including the protective device is never again used if the abnormal charging current is detected once. Also, there arises such a problem that the efficiency is deteriorated by heat of the resistance component of the current fuse. Further, there arises such a problem that the addition of the current fuse causes the number of parts to increase and the mounted area increases, resulting in the prevention of lowering the costs, decreasing the size and weight thereof.